Their bleak update to the case comes one day after officers arrested Tanner James Prevost, 21, and charged him with second-degree murder in connection with Thomson's death, and mere weeks after members of the victim's family gave a public plea for help solving his disappearance.

Police said on the day Thomson, 21, was last seen alive -- July 21, 2012 -- he met a man and woman near River Avenue and Osborne Street.

The three strangers bought liquor at the area's Liquor Mart and went to drink it at an area along the riverbank under the Osborne Street Bridge.

At some point, Thomson and the man he had just met became involved in a "serious physical altercation," Const. Jason Michalyshen said.

His body was dropped in the Assiniboine River, and his attacker fled. Thomson's body has not been found, police said.

Since Thomson vanished, missing-persons investigators have made several requests for the public's help in finding the well-liked and friendly young man.

While police wouldn't be specific Tuesday about when homicide officers got involved, it's clear the overall nature of the case changed following an emotionally charged Nov. 28 press conference where Thomson's father, Robert Thomson Sr., asked for people to come forward with any lead that may bring the family closure.

Police also released a photo of Bobby Thomson taken on the day he was last seen but declined to confirm where it came from.

On Dec. 11, police released a composite sketch of a man they said looked like the person Thomson was seen with in Osborne Village.

"Things evolve. They're very fluid," Michalyshen said Tuesday of the progression in the case.

He added it wasn't uncommon for the missing-persons and homicide units to share information.

Michalyshen confirmed Tuesday they've identified the woman who accompanied the men to the riverbank and she is considered a witness in the case. She is not facing charges.

The homicide unit is speaking with colleagues to see what efforts could be made to try and find Thomson's remains. A river search is a likely possibility, police indicated.

Prevost was arrested while in custody on unrelated charges, court records show.

He's been locked up since October. He has no criminal record but is in custody pending a lengthy list of unrelated charges amassed since late April 2012.

The most serious among them relate to an alleged domestic assault in which he's accused of slamming his then-girlfriend's head into a bus shelter. Prevost denies this happened.

He was wanted on two arrest warrants and was suspected of skipping out on a supervised residential bail program around the time Thomson went missing.

He was ultimately arrested in September 2012 and bailed out again to attend what was described as a "last-ditch" residential treatment program.

He's suspected of absconding from that by December 2012 and going on the lam until last fall, when police learned he was staying at a friend's Lansdowne Avenue home and arrested him.

A message left by a reporter for Robert Thomson Sr. went unreturned Tuesday.

Michalyshen said he spoke with the victim's family members briefly before publicly announcing an arrest was made.

There's no question they're still grieving Thomson's loss, he said, describing them as "wonderful people."

"I got the sense that maybe there was a bit of relief," said Michalyshen. "However, they're still facing a lot of challenges and no question, they're still mourning."

"I feel sorry for these people," said Floyd Wiebe.

Wiebe's son, T.J., was publicly declared missing by police Jan. 21, 2003, after the 20-year-old hadn't been seen for more than two weeks.

Days after police came forward to ask for help finding him, it was discovered T.J. had been brutally murdered and his body left covered in snow on the side of a private road outside Winnipeg.

Wiebe suspects Thomson's kin would have considered the possibility Bobby had died over the many months they've spent searching for him.

"They may have processed that already," he said. "But they would have had a glimpse of hope until those cops walked into the room and announced the murder charge," said Wiebe.